1. Field of the Invention
This invention generally relates to communication systems. More specifically, the invention relates to scaling and rotating a complex symbol by a complex gain extracted from another complex symbol. The complex gain may be extracted from a pilot signal, complex conjugated, and then applied to the output of a filter matched to a complex traffic channel signal. Embodiments of the invention may be configured to process a spread spectrum signal, such as a Code Division Multiple Access (“CDMA”) signal or a Wideband CDMA signal (“WCDMA”).
2. Discussion of Related Art
Communication systems, such as spread spectrum CDMA systems used in the cellular telephony industry, use a variety of modulation schemes. Examples of such modulations include Quadrature Amplitude Modulation (QAM), Quadrature Phase Shift Keying (“QPSK”) and Bi Phase Shift Keying (“BPSK”). Those skilled in the art readily understand QPSK and BPSK modulation schemes. These spread spectrum systems transmit signals that convey digital data spread across a band of the radio frequency (“RF”) spectrum. For example, transmitted CDMA signals convey digital data that is interspersed or “spread” with pseudo random (“PN”) codes at a data rate that is typically much greater than the underlying data rate. As such, the underlying data is spread over a larger spectrum than it otherwise would be.
As used in the cellular telephony industry, a base station will transmit a CDMA signal to a subscriber unit, such as a cell phone. Such signals may be represented using polar or Cartesian representations. Thus, the complex gain of a signal or symbol may be described in terms of complex gain and phase rotation, or in terms of a linear transformation on the in-phase (I) and quadrature-phase (Q) components of the signal or symbol. For example, the complex conjugate of a complex gain G=|G|eiφ=GI+iGQ may be applied to a complex symbol X=|X|eiθ=XI+iXQ to produce the symbol Y=|Y|eiψ=YI+iYQ, where Ψ=θ−φ. The resulting product may be expressed byY=G*X=|Y|eiΨ=|G∥X|eiΨY=YI+iYQ=|G|XI cos φ+|G|XQ sin φ+i(|G|XQ cos φ−|G|XI sin φ).
The CDMA signal may be a complex signal comprising I and Q components. Digital data may be transmitted through either or both of these I and Q components. The data is despread at the receiving subscriber unit by combining the I and Q with the same PN code used to spread the data. The subscriber unit thereby extracts the data and converts it to a desired format, such as voice. In the cellular telephony example, extraction of the data may include combining the data with an orthogonal code sequence, such as a Walsh Code sequence. For example, the underlying data may include a plurality of channels. As used herein, a channel refers to an encoded bit spread by an orthogonal code, such as the Walsh code. The same orthogonal code used to encode the data at the transmitter side is subsequently used to extract the data at the receiver side.
The transmitter may send a reference signal known as a pilot signal. This pilot signal is typically used as a phase reference for the received data. For example, the pilot signal may be used to adjust or rotate the phase of the QPSK signal to a predetermined phase quadrant so that the underlying data may be correctly recovered. Since the pilot signal typically conveys no data and uses a zero sequence for encoding, the pilot signal should lie in a single predetermined phase quadrant. Accordingly, if the pilot signal is detected in an incorrect phase quadrant, the pilot signal, and thus the underlying data, can be rotated in phase to the correct phase quadrant.
A simple representation of Y=YI+iYQ is
      [                                        Y            I                                                            Y            Q                                ]    =                            G                    ⁡              [                                                            cos                ⁢                                                                  ⁢                ϕ                                                                    sin                ⁢                                                                  ⁢                ϕ                                                                                                          -                  sin                                ⁢                                                                  ⁢                ϕ                                                                    cos                ⁢                                                                  ⁢                ϕ                                                    ]              ⁡          [                                                  X              I                                                                          X              Q                                          ]      
Typically, the angle φ is referenced to the angle zero, and the rotation by −φ is used to rotate X toward the real axis. Thus, the real and imaginary parts of Y, expressed byYI=|G∥X|cos(θ−φ)=|G|XI cos φ+|G|XQ sin φ=GIXI+GQXQ YQ=|G∥X∥sin(θ−φ)=|G|XQ cos φ−|G|XI sin φ=GIXQ−GQXI may be used for detection in coherent communications. If the transmitted symbols are real (such as may occur in a pilot transmission), then only the real part may be used.
Techniques for rotating the phase, also referred to as carrier phase recovery, exist in the art. However, such techniques operate at a symbol level, as they are used for data recovery of a single channel within the underlying data. For example, one prior-art system combines a Walsh code sequence with each of the I and Q components of a signal immediately after those components are despread. This system is efficient in determining the complex amplitude of a channel selected for data recovery. However, to determine the complex amplitude of multiple channels, a plurality of Walsh codes is needed. The additional Walsh codes contribute to the complexity of the system because more components (e.g., filters, accumulators and multipliers) are needed. Additionally, since each channel requires a multiplication of the Walsh code to the I and Q components, more processing power is consumed due to the extraction of multiple channels. Examples of a system used to rotate phase and thereby extract data of a single channel are illustrated in U.S. Pat. No. 5,506,865 (issued Apr. 9, 1996; the “'865 patent”) and U.S. Pat. No. 6,396,804 (issued May 28, 2002; the “'804 patent”).
While prior-art systems are particularly useful in extracting data of a single channel, they offer substantially no interference-cancellation capabilities. Interference cancellation can be useful in spread-spectrum systems, such as the CDMA systems of cellular telephony described above, to reduce interference from other sources. Examples of such interfering sources include leakage of energy from other channels that degrade the quality of the selected channel. Leakage may result from calculation errors in down conversion and/or demodulation of a received signal. Accordingly, interference-cancellation receivers can be used to assist in the isolation and extraction of individual channels within spread spectrum signals. Prior examples of interference-cancellation receivers include U.S. Pat. No. 5,930,229 (issued Jul. 27, 1999) and U.S. Pat. No. 5,872,776 (issued Feb. 16, 1999); these examples may provide useful background information.
The phase rotation of some prior-art systems is particularly inefficient for interference cancellation purposes because of the multiple code sequences necessary for extracting data from each channel within a given signal. For example, in isolating an intended channel, each of the unintended channels is considered as a potential interferer with the intended channel; therefore, the unintended channels must be identified so that any potential interference can be countered and the data may be correctly extracted from the intended channel. Since each channel requires its own code sequence for data extraction, a receiver's complexity is inherently increased through the addition of various components necessary for the data extraction.